Want to run query and calculate a field called "tot_hours" in the Task entity?

[worldmiros]

Code:
$sql = "SELECT * FROM task";
$res = $conn->query($sql);
$len = $res->num_rows;

if($len > 0){
while($row = $res->fetch_array(MYSQLI_ASSOC){
if($row['status'] == "Completed" && empty($row['tot_hours'])){
$sql = "UPDATE task SET tot_hours = TIME_TO_SEC(TIMEDIFF(date_end,date_start))/3600 WHERE status = 'Completed'";
}
else{
continue;
}
}
}

(Not familiar with PDO, but something like this in application/espo/core/utils/cron/job.php)

protected function updateHours(){
$pdo = $this->getEntityManager->getPDO();
$query = "SELECT * FROM task";
$sth = $pdo->prepare($query);
$sth->execute();

while($row = $sth->fetch(PDO::FETCH_ASSOC)){
if($row['status'] == "Completed" && empty($row['tot_hours'])){
$sec_query = "UPDATE task SET tot_hours = TIME_TO_SEC(TIMEDIFF(date_end,date_start)/3600 WHERE status = 'Completed' ";

$fst = prepare($sec_query)->execute();
else{
continue;
}
}
}

Not sure where to execute the code? Need assistance on how and where to execute the code above to calculate the difference of datetime. Maybe run it as a cron job.